def filter(s: str, target: list) -> str:
    target = list(map(chr, target))
    return "".join([c for c in s if c in target])

def filter_chars(s: str) -> str:
    english = list(range(ord('A'), ord('Z') + 1)) + list(range(ord('a'), ord('z') + 1))
    return filter(s, english)

if __name__ == "__main__":
    s = input("字符串：")
    print("结果为：{}".format(filter_chars(s)))